As is known, electronic devices for power and control purposes are conveniently used in MV apparatuses (e.g. reclosers, circuit breakers, contactors, disconnectors) to operate the loiters. As an example, a MV switching apparatus typically comprises an electronic P&C device for driving the actuator of the switching apparatus and providing control functions for properly managing the operation of the actuator, protection functions against possible over-currents circulating in the actuator and/or diagnostic functions to check the functionalities of the actuator and/or the internal circuits.
As is known, some MV applications (typically in certain outdoor installations) operate in critical environmental conditions and may be subject to very low temperatures, even lower than −40° C.
As commonly available industrial electronic equipment is typically designed for operating at temperatures higher than the rated minimum operating temperature of −40° C., a P&C device for use in these MV applications is generally provided with or operatively associated to an electric heater that is powered to maintain the operating temperature of higher values. Traditional P&C devices of the type described above have some drawbacks.
In currently available solutions, when the electric power supply is interrupted for some reasons (e.g. due to a fault), the electric and electronic equipment of the P&C device (including the electric heater) is switched-off.
The internal temperature of the P&C device may thus drop to values lower than the rated minimum operating temperature.
Thus, as the power supply is restored, the electronic equipment of the P&C device may start operating at temperatures out of the nominal operating range for which it was designed (cold start).
These circumstances may easily give raise to faults and/or determine improper operations of the P&C device.
On the other hand, some traditional P&C devices are not provided with means to automatically switch-off the electric heater, when the internal temperature increases above a given threshold value.
Some electronic equipment of the P&C device may thus be subject to over-heating phenomena when the environmental temperature rises for some reasons. This may cause faults or a remarkable reduction of the useful life.
In other cases, a thermostat is provided to automatically switch-off the electric heater.
This solution does not ensure satisfactory performances in terms of reliability.
In fact, commonly available industrial thermostats are sometimes subject to configuration errors or faults that may jeopardize the quality of their performances.
A possible solution to overcome most of the drawbacks described above would be to use military electronic equipment for the realization of the power and control device.
As is known, electronic equipment of this type ensures outstanding and reliable performances and is generally rated for a minimum operating temperature of −55° C. or lower.
Unfortunately, this solution entails high industrial costs for the realization of the P&C devices.